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Lerma(lār`mä), river, c.350 mi (560 km) long, rising in Mexico state, central Mexico. It flows NW and W through Guanajuato state to Lake Chapala, crossing the part of the central plateau known as the Anáhuac. The river draining the lake and flowing NW through Jalisco state to the Pacific Ocean is generally called the Rio Grande de Santiago (c.200 mi/320 km long) but it is considered a continuation of the Lerma. The river system is extensively used for irrigation and hydroelectric power.
(Rio Herma), a river in Central Mexico. It is part of an important water system that also includes Lake Chapala and the Rio Santiago, with the latter emptying into the Pacific Ocean. The entire system is approximately 1,000 km long and has a basin area of 124,800 sq km. The Lerma itself (from its source to Lake Chapala) is 510 km long and has a basin area of 38,500 sq km. The river originates from the Lerma Lagoon in the Toluca Valley, southwest of Mexico City, at an elevation of 2,600 m above sea level. It flows through a densely populated area of Central Mexico (Bajío), where there are large reservoirs on the river (Salis and others) whose waters are used for irrigation and energy. The Lerma is fed by rain; the regime of the river is irregular. The mean flow rate is 80 cu m per sec.